1. Field of the Invention
The present invention relates in general to electrochemical detection, and is more particularly directed to an "all solid state" ion selective electrode having a unique sensing element of triple layer construction. Other related embodiments of the invention include new solid electrolyte compositions for the selective determination of mercury and thallium ions whereby these compositions may be utilized in conjunction with the "all solid state" electrode disclosed herein.
2. Description of the Related Art
Ion selective electrodes (ISEs) have found wide practical application in the field of electrochemistry. Capable of detecting and measuring the concentration of a particular ionic species in solution, ISEs provide a useful analytical technique for the single measurement or continuous monitoring of flowing samples in solution. The applications of ISEs are numerous including biomedical research, clinical testing, industrial pollution testing and chemical-process control.
Solid state ISEs have been developed in the art which generally comprise a solid membrane as the sensing element which possesses ionic conductivity and is highly selective to a specific ionic species. In operation, one surface of the membrane is immersed in a sample solution of ions for which it is selective whereby a potential develops across the membrane surface at the interface of the solution and the membrane. This potential varies with the concentration of ions in solution and its magnitude is measured as a voltage. By comparing the voltage generated at the sensing membrane surface with that of a standard electrode of unvarying potential, it is possible to calculate the unknown solution's ion concentration.
A number of solid state membrane electrodes are patterned after glass electrodes consisting of the selective membrane affixed to the lower opening of a plastic barrel and having an inner electrolyte filler solution and reversible internal reference electrode sealed within. One problem encountered with ion-selective electrodes constructed of an internal reference electrode and internal solution is that weaknesses in the membrane seals can provide low resistance short-circuit pathways for ionic flow resulting in inaccurate measurements of membrane potential, particularly where constructed as a relatively small microelectrode such as that used in biomedical testing. In addition, those electrodes having an internal reference electrode and solution are relatively delicate instruments not particularly well suited for industrial or other practical field uses.
In order to overcome these problems, "all solid state" membrane electrodes have been developed which eliminate the inner filler solution and reference electrode and instead include a direct electrical contact to the inner surface (the surface not in contact with the sample solution) of the selective membrane. The membranes of these electrodes commonly include a polycrystalline pressed pellet of solid electrolytes and have a silver or gold plating on the membrane inner surface for electrical contact to a volt meter.
A variety of electrolyte compositions for use in "all solid state" membranes are known in the art that are selective to a specific ion. For example a pressed-pellet membrane comprising precipitates of silver sulfide is selective for Ag.sup.+ or S.sup.2- ions. In addition, silver sulfides can be used in conjunction with silver halides or other sulfide salts to provide a whole series of heavy metal ISEs. [See e.g. U.S. Pat. No. 3,591,464 to Frant disclosing materials for the detection Cl.sup.-, Cu.sup.2+, Br.sup.-, I.sup.-, SCN.sup.-, Cd.sup.2+, and Pb.sup.2+ ]. In order to expand the scope of already known and commercially available ion detectors, those in the art continue to seek new membrane compositions and materials that exhibit high selectivity toward a particular ionic species. To this end, in one embodiment of the present invention a novel membrane material is provided for the selective measurement of thallium ions. In another embodiment, a novel composition capable of detecting mercury is disclosed.
Although the "all solid state" electrode constructions heretofore developed in the art are relatively sturdy and particularly convenient for continuous industrial measurements and field use applications, it has proven difficult to provide the direct electrical contact as is required to the membrane surface at a fixed potential. This problem principally lies in the transition of charge transfer from ionic conductivity in the membrane to electronic conductivity in the electrical contact. Polarization distortion or blocking can occur at the junction between the metallic conductor and the sensing membrane which contributes or detracts from the potential at the membrane surface. This distortion affects an erroneous measurement of potential difference between the sample and standard reference resulting in inaccurate and inconsistent ion concentration measurements. To ease the transition of ionic conductivity and minimize this polarization effect, the present inventors heretofore developed a membrane construction of three layers: a first sensor layer comprising the electrolyte composition for contact with the sample solution, a middle layer comprising a uniform mixture of the electrolyte composition and silver powder, and a third layer of silver powder connected to a wire contact. [USSR Patent No. 630576 to Vlasov & Ermolenko]. The three layer system minimizes polarization effects to some extent, but the problem is not eliminated altogether, and meter drift and unreliability of measurement is still encountered.
It is therefore a primary object of the present invention to provide an improved all solid state ion selective electrode for detecting and measuring the presence of ions in solution.
It is another object of the present invention to provide an improved all solid state electrode having a relatively high level of stability of potential across the sensing membrane.
Another object of the present invention is to provide an all solid state ion selective electrode that is durable and that exhibits effectiveness for extended in use service.
A further object of the present invention is to provide an all solid state ion selective electrode that exhibits high precision and reliability of measurement.
It is also an object of the present invention to provide an all solid state ISE which permits on-stream monitoring of ion activity.
Yet another object of the present invention is to provide an all solid state ISE having a sensing membrane constructed to achieve all the objects heretofore set forth.
A further object of the present invention is to provide a novel electrolyte composition that is highly selective to mercury.
It is yet a further object of the present invention to provide a novel electrolyte composition selective for thallium.
These and other objects are achieved by an improved "all solid state" ion selective electrode having a unique sensing element comprising a polycrystalline membrane of triple-layer construction. The first sensor layer of the membrane is composed of a solid electrolyte composition selective to a specific ion in solution, the intermediate or middle layer is comprised of a mixture of the aforesaid electrolyte composition of the sensor layer and a finely divided electronic conducting material, and the third electrical contacting layer is composed principally of the aforementioned material having electronic conductivity. The improvement being that the intermediate (middle) layer has a gradient of component concentration such that a substantial amount of the electrolyte composition is presented in the middle layer closest the sensor layer and a substantial amount of the electronic conducting material is presented in the middle layer closest the electrical contacting layer. The layers are pressed together under pressure and in the presence of heat to provide a substantially non-porous polycrystalline ceramic membrane. This unique construction provides an electrode that has an extremely high level of stability of potential across the surface of the membrane and which exhibits excellent precision and reliability of measurement during long term operation.
Although the inventors do not wish to be limited by theory, it is hypothesized that the gradient of concentration in the intermediate layer enables relatively continuous and "easy" passage from pure ionic conductivity at the surface of the sensor layer, where the potential has been created, to pure electronic conductivity at the electrical contacting layer.
In a related embodiment of the invention, a novel solid electrolyte composition is provided for the determination of mercury, wherein the composition comprises a mixture of thallium iodide (TlI) and silver iodide (AgI). A novel electrolyte composition comprising Ag.sub.8 HgS.sub.2 I.sub.6 and a method of preparing the same is also provided, wherein the composition is useful for the measurement of mercury. These new electrolyte compositions may be used in conjunction with the sensing element of the present invention or with any conventional ISE now known or later developed in the art.